The present invention relates to a battery structure for preventing leakage of electrolyte or the like out of a battery.
In recent years, development in high performance and compactness of electronic equipments such as video cameras and stereo headphones has been remarkable, and demand for high capacity of a battery to be used as a power source for such electronic equipments has accordingly been increased. A manganese battery is conventionally used as such a battery, and an alkaline battery employing an alkaline electrolyte such as potassium hydroxide is widely used to obtain a larger capacity.
In general, a battery has a sealed structure, and an increase in internal pressure in the battery occurs for some reason. For example, in case of storing an alkaline battery for a long period of time, there is a possibility that zinc in a negative electrode corrodes to generate a hydrogen gas, which causes the increase in the internal pressure. When the internal pressure increases, the sealed structure is broken by the increased pressure to result in loss of a function as the battery or damage to peripheral equipments.
To prevent such a trouble, it is known that the battery is provided with a structure functioning as a relief valve adapted to relieve an excessive increase in the internal pressure. Such a structure in an alkaline battery will be described with reference to FIG. 1.
The alkaline battery shown in FIG. 1 is generally constituted of a cylindrical metal housing 51, a positive electrode 52 composed of manganese dioxide and graphite, and a gelled negative electrode 53 composed of granulated zinc, an aqueous solution of potassium hydroxide, a thickener, etc. The positive electrode 52 and the negative electrode 53 are contained in the metal housing 51 and are separated from each other by a separator 54 formed of nonwoven fabric. The metal housing 51 is integrated with the positive electrode 52 to constitute a positive electrode can. The metal housing 51 has an opening at a lower end thereof as viewed in FIG. 1. A sealing member 56 for sealing this opening is provided at the lower end of the metal housing 51, and a reinforcing member 57 for reinforcing the sealing member 56 is assembled with the sealing member 56 on the outside thereof. Further, a cover 58 serving as a negative electrode terminal is provided on the outside of the reinforcing member 57 so as to cover the opening of the metal housing 51. The cover 58 is retained at its outer periphery to the metal housing 51. The sealing member 56 is formed at its central portion with a through hole 56a, and a nail-like current collecting pin 55 is press-fitted in the through hole 56a. The reinforcing member 57 is also formed with a central through hole 57a, and a cylindrical portion 56b of the sealing member 56 is engaged with the central through hole 57a.
The sealing member 56 is formed of resin such as plastic (e.g., nylon), and it is formed with a thin-walled portion 56c functioning as a relief valve. In the case that an internal pressure in the metal housing 51 sealed by the sealing member 56 increases, the thin-walled portion 56c is broken to allow escape of an internal gas from the broken thin-walled portion 56c to the outside of the battery, thus preventing abnormal increase in the internal pressure. To allow the relief of the internal gas out of the metal housing 51, there are formed vent holes 57a and 58a through the reinforcing member 57 and the cover 58, respectively.
Although possible breakage of the battery due to an increase in the internal pressure can be effectively prevented by providing the thin-walled portion 56c in the sealing member 56 formed of plastic or the like, there is a problem in the battery having the above-mentioned structure. That is, in case of storing the battery for a long period of time, the cylindrical portion 56b of the sealing member 56 flaws or cracks at a central portion thereof (for example, in the vicinity of a shoulder 56d of the cylindrical portion 56b) to cause leakage of electrolyte or the like out of the metal housing 51. It is considered that the occurrence of the flaws or cracks is primarily caused by an internal stress to be generated in the sealing member 56 because of expansion of a diameter of the through hole 56a of the sealing member 56 upon press-fitting of the current collecting pin 55 into the through hole 56a.